Gel for retarding water flow

ABSTRACT

A polyvinyl alcohol-aldehyde gel having a high water content and low degree of crosslinking is provided which has improved stability. The gel can be used to retard the flow of water in subterranean formation encountered in hydrocarbon production. One embodiment of the invention provides a method of forming the gel in-situ in the pores of subterranean zones thereby retarding or blocking the flow of water therein.

TECHNICAL FIELD

This invention relates to gels, methods of forming gels, and uses forgels. A polyvinyl alcohol-aldehyde hydrogel is provided which is usefulfor immobilizing large volumes of earth or water. The gel can be usedfor reducing the permeability of soils and subterranean formations tothe flow of water or brines. The gels of this invention are particularlyvaluable in retarding the flow of water in hydrocarbon production from awellbore, or from solar ponds.

BACKGROUND OF THE INVENTION

The recovery of hydrocarbons, both liquid and gaseous, from subterraneanzones has frequently resulted in the simultaneous production of largequantities of water. In some cases, even though substantial flows ofhydrocarbons have been shown, water production is so great and waterdisposal costs so high, that hydrocarbon production is not economical.Such water production has in some cases been disposed of in an abandonedor dry well by separating such water from the hydrocarbons andreinjecting the separated water into such wells. Where a disposal wellis not available nor near the producing well, pipelining the waterproduct over a long distance to a disposal site can become so costlythat it renders the well noncommercial. Even if a disposal well is closeby, the disposal cost can still be very expensive. Therefore it isdesirable to find a way to reduce or shut off the flow of water whilepermitting hydrocarbon production to continue.

Thus, it is well known that the production of large amounts of waterfrom hydrocarbon producing wells is a major expense item in the overallhydrocarbon recovery cost. It is not uncommon for an oil well to producean effluent which is 60-99% by volume water and only 1-40% by volumeoil. In such situations, the major part of the pumping energy isexpended in lifting water from the well, a cost which the producer wouldlike to avoid if possible. The effluent must then be subjected to acostly separation procedure to recovery water-free hydrocarbons. Thefoul water separated therefore also presents a troublesome and expensivedisposal problem. Consequently, it is desirable to decrease the volumeof water produced from hydrocarbon wells. It is, of course, desirable tobe able to achieve this objective and at the same time not materiallyaffect the hydrocarbon recovery rate. However, where the volume of wateris very high, e.g. 80 to 99% water, and only 1-20% oil, even substantialreduction in hydrocarbon production can be tolerated if water productioncan be substantially reduced.

One such method of reducing the flow of water has been described in U.S.Pat. No. 3,762,476 wherein a first aqueous polymer solution selectedfrom the group consisting of a polyacrylamide, a partially hydrolyzedpolyacrylamide, a polysaccharide, a carboxymethylcellulose, a polyvinylalcohol, and polystyrene sulfonate, is injected into a subterraneanformation. Thereafter, a complexing ionic solution of multivalentcations and retarding anions, and which also comprises aluminum citrate,is injected into the subterranean formation. The multivalent cations areselected from the group consisting of Fe(II), Fe(III), Al(III), Ti(IV),Zn(II), Sn(IV), Ca(II), Mg(II), Cr(III), and the retarding anions areselected from the group consisting of acetate, nitrilotriacetate,tartrate, citrate, phosphate. Brine is then injected followed by asecond slug of an aqueous polymer solution which can be the same ordifferent from the first aqueous polymer solution. In any event, thecomplexing ionic solution of multivalent cations and retarding anions iscapable of gelling both the first and second aqueous polymer solution.

Water produced from a well bore can come from the infiltration ofnaturally occuring subterranean water as described above, or the watercan come from injected water put into the formation in those hydrocarbonrecovery processes which utilize water flooding. U.S. Pat. No. 4,098,337discloses a method for forming a hydroxymethylated polyacrylamide gel,in situ, to reduce the permeability of a thusly treated zone where thewater flood method of oil recovery is employed. In this case the gel wasformed in situ by the injection of an aqueous polyacrylamide solutionand an aqueous formaldehyde solution.

In a water flood operation it can be desirable to treat the waterinjector wells with a polymer gel forming solution to control and/orredirect the water flow profile. Such treatment can prevent channelingof water at the injector well and/or control or redirect the water flowthrough regions of varying permeability.

Although polyacrylamide-based gels can be effective in retarding waterproduction or flow in some subterranean formations polyacrylamide-basedgels will not be stable or effective in all formations. In general,polyacrylamide-based gels will work satisfactorily in formations havinga temperature below about 65° C. Above about 65° C.,polyacrylamide-based gels become very sensitive to hardness of thebrines, especially where hardness is above about 1000 ppm. The hardnessof the water becomes a more detrimental factor the higher thetemperature, thus for very hot regions lower hardness levels can rendermany gels ineffective. Formations which have a higher temperature,hardness, or total dissolved solids content above the aforementionedranges usually are not capable of being successfully treated withpolyacrylamide-based polymers to retard the flow of water.

In many hydrocarbon producing wells temperatures of 80° C. or higher areoften encountered. Formation waters frequently have hardnesses whichexceed 1000 ppm. It is therefore desirable to develop a gel which can beused to retard or block the flow of water in subterranean formationshaving a temperature of 65° C. or higher, and a water hardness of 1000ppm or higher.

This invention addresses this problem and provides a means of treatingsuch wells with a polyvinyl alcohol based gel which can overcome many ofthe short comings of prior art gels.

Polyvinyl alcohol gels have been used to protect well casings fromcorrosion. U.S. Pat. No. 2,832,414 discloses such a method wherein anaqueous solution of a water soluble polyvinyl alcohol which is capableof forming a gel if maintained in a quiescent state, is pumped into theannular space between the casing and the wall of the bore hole. Afterallowing the polymer to remain quiescent over a period of time a gel isformed. The thusly formed gel prevents the intrusion of formation waterinto the annular space thereby reducing corrosion of the metal casing.Apparently, no crosslinking agent is employed and for that reason is notbelieved that this particular gel would be useful. Furthermore, in U.S.Pat. No. 2,832,414 the gel is used to fill a relatively large cavitycompared to the cavity volume of a typical pore in a subterraneanformation associated with hydrocarbon production from a well bore.

Studies of the macroscopic changes in polyvinyl acetate gels that occurupon removal from swelling equilibrium with isopropyl alcohol werereported in the Journal of Colloid and Interface Science, Vol. 90, No.1, November 1982, pages 34 to 43. These studies were conducted usingfilms of gels having various degrees of crosslinking and polymerconcentration. The polyvinyl acetate gels were formed from precursorpolyvinyl alcohol gels that were crosslinked with glutaric dialdehydewhich were then converted to acetate gels by polymer homologousacetylation.

SUMMARY OF THE INVENTION

By the term "aldehyde" as used herein is meant a monoaldehyde, adialdehyde, a polyaldehyde, and any of the former whether substituted orunsubstituted. Preferably the aldehyde contains two functional groupssuch as a dialdehyde or a substituted monoaldehyde. By substitutedmonoaldehyde as used herein is meant to include unsaturatedcarbon-carbon bond as well as substitution of functional groups.Non-limiting examples of substituted monoaldehyde are acrolein andacrolein dimethylacetal. Polyaldehydes can be used and may in some casesbe more desirable, however, polyaldehydes are not as availablecommercially as dialdehydes and as a consequence use of polyaldehydesmay not be practical.

Non-limiting examples of dialdehyde crosslinking agents are glyoxal,malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde,terephthaldehyde. Non-limiting examaples of dialdehyde derivatives areglyoxal bisulfite addition compound

    Na.sub.2 HC(OH)SO.sub.3 CH(OH)SO.sub.3

glyoxal trimeric dihydrate, malonaldehyde bisdimethylacetal,2,5-dimethoxytetrahydrofuran, 3,4-dihydro-2-methoxy-2H-pyran, andfurfural. Acetals, hemiacetals, cyclic acetals, bisulfite additioncomponds, shiff's bases or other compounds which generate dialdehydes inwater, either alone or in response to an additional agent such as anacid or a condition such as heat are also meant to be included in theterm "aldehyde" as used and claimed herein.

Non-limiting examples of monoaldehyde with a second functional group inaddition to the aldehyde group are acrolein and acrolein dimethylacetal.

Non-limiting examples of polyaldehydes are polyacrolein dimethylacetal,addition products of acrolein for example, ethylene glycol plusacrolein, and glycerol plus acrolein.

By the term "acidic catalyst" as used herein is meant a substance whichis a proton donor or a substance which in its environment will form orbecome a proton donor. All acids are operable as an acidic catalyst inthis invention, for example, Bronsted acids such as mineral andcarboxylic acids, or Lewis acids. Non-limiting examples of a Lewis acidare zinc chloride, ferrous chloride, stannous chloride, aluminumchloride, barium fluoride, and sulfur trioxide. Some of these chemicalshydrolyse in water to produce metal oxides or hydroxides and HCl or HF.The rate of hydrolysis of many Lewis acids is dependent on temperatureand the other dissolved compounds in the solution. The rate ofproduction of the acidic catalyst, HCl, from some of the above Lewisacids determines the rate of gel formation.

A delayed action catalyst is a substance which is not acidic in and ofitself, but which generates an acidic catalyst slowly on interactionwith water at the temperature of interest. For example, the rate ofgeneration of the acid in oil well usage is usually controlled by thereservoir temperature experienced during the in-situ gel formation. Inmany applications the rate of acidic catalyst generation or release canbe controlled by the gel-forming fluid formulation to range from a fewminutes to a few days or more.

In one embodiment of this invention the acid catalyst can be a twocomponent system, for example, a two component delayed action catalystcan comprise a first component which will react with a second component,to form an acidic catalyst. A non-limiting example of such a twocomponent delayed action catalyst is sodium persulfate and a reducingagent. In such a delayed catalyst system the sodium persulfate reactswith the reducing agent to produce sulfuric acid. In another twocomponent delayed action catalyst system the reaction product of the twocomponents can react with water to form the acidic catalyst.

The acidic catalyst and/or delayed action catalyst must, of course, havesome solubility in water. However, in some oil field usages the partialsolubility of the acidic catalyst in the oil product can be advantageousif treatment is to include subterranean zones containing both oil andwater. The fraction of the acidic catalyst or delayed action catalystwhich dissolutes in oil will, of course, not be available to catalyzethe gel formation reaction in such zones of high oil content;consequently such oil-water zones will not be blocked by gel formationto the same extent as those zones with little or no oil present.

Non-limiting examples of delayed action catalysts are methyl formate,ethyl formate, methyl acetate, ethyl acetate, glycerol monoacetate oracetin and glycerol diacetate or diacetin.

Laboratory tests conducted on core samples have shown that diacetinhydrolysis more slowly than methyl formate at all temperatures includingthe higher temperatures. Therefore in some embodiments of this inventionwhere subterranean formations having higher temperatures areencountered, diacetin or acetin because of their slower rate ofhydrolysis are used to provide a longer time for crosslinking reactionsto occur and hence provide a longer time for the gelling forming fluidsto penetrate into the pores of such subterranean zones before gelationoccurs. Non-limiting examples of delayed action catalyst and theiracidic catalyst product are:

    ______________________________________                                        Delayed Action Catalyst                                                                          Acidic Catalyst Product                                    ______________________________________                                        Methyl formate     Formic acid                                                Glycerol diacetate Acetic acid                                                Sodium persulfate  Sulfuric acid                                              Sodium dadecyl sulfate                                                                           Sulfuric acid                                              Methyl methane sulfonate                                                                         Methylsulfonic acid                                        Sodium triiodide/sodium                                                                          Hydroiodic acid                                            bisulfate/water                                                               ______________________________________                                    

Therefore, delayed action acidic catalysts can be esters which slowlyhydrolyze in water, the rate of hydrolysis being dependent ontemperature and initial pH. Other delayed action catalysts are theanalogs of esters and acids such as sulfones, xanthates, xanthic acids,thiocyanates, and the like. In some of these examples, hydrolysisproduces an acid catalyst which speeds the crosslinking reaction and analcohol which does not affect gel formation. An example of a delayedaction acidic catalyst is methyl formate which is influenced by theenvironment with respect to the rate of formation of acid. For example,the higher the temperature, the faster methyl formate will hydrolyze andgenerate formic acid.

By the term "Bronsted acid" as used herein is meant a chemical which canact as a source of protons. By the term "Lewis acid" as used herein ismeant a chemical that can accept an electron pair from a base. By theterm "delayed action acid" as used herein is meant any acidic catalystwhich makes available or generates donor proton over a period of time orafter an initial period of time either as a consequence of itscharacteristic or the characteristics of the environment in which it isused.

By the term "gel" as used herein is meant a chemically crosslinkedthree-dimensional elastic network of long-chain molecules with a certainamount of immobilized solvent (diluent) molecules.

By the term "PVA-aldehyde gel" as used herein is meant a chemicallycrosslinked three-dimensional elastic network of long-chain molecules(frequently referred to herein as the "first substance") selected fromthe group consisting of a polyvinyl alcohol, a polyvinyl alcoholcopolymer, and mixtures thereof, crosslinked with an aldehyde, andcontaining a certain amount of immobilized and chemically bound watermolecules.

Non-limiting examples of polyvinyl alcohol copolymers are polyvinylalcohol-co-crotonic acid, polyvinyl alcohol-co-acrylic acid, polyvinylalcohol-co-methacrylic acid, polyvinyl alcohol-co-vinylpyridine, andpolyvinyl alcohol-co-vinylacetate, the latter of which is frequentlypresent in small amounts in commercial grade polyvinyl alcohols.

DESCRIPTION OF THE GELS

There is provided by this invention a gel formed by reacting, in thepresence of an acidic catalyst, a first substance selected from thegroup consisting of polyvinyl alcohol, a polyvinyl alcohol copolymer,and mixtures thereof, with a second substance comprising an aldehyde,and water, wherein such water provided at least about 95% of the weightof the gel. In another embodiment such water provides at least about 96%of the weight of the gel. In still another embodiment such waterprovides at least about 97% of the weight of the gel. In a preferredembodiment such water provides at least about 98% or about 99% of thegel.

There is also provided by this invention a gel formed by reacting, inthe presence of an acidic catalyst, a first substance havingcrosslinkable sites selected from the group consisting of polyvinylalcohol, polyvinyl alcohol copolymer, and mixtures thereof; a secondsubstance comprising an aldehyde, the amount of the aldehyde beingadjusted so that stoichiometrically no more than about 8% of thecrosslinkable sites of the first substance can be crosslinked with thealdehyde; and water. In one embodiment the amount of said aldehyde isadjusted so that stoichiometrically no more than about 4% of thecrosslinkable sites of the first substance can be crosslinked with thealdehyde. In a preferred embodiment the amount of the aldehyde isadjusted so that stoichiometrically no more than about 2% of thecrosslinkable sites of the first substance can be crosslinked with thealdehyde. In another preferred embodiment the amount of aldehyde isadjusted so that stoichiometrically no more than about 1% of thecrosslinkable sites of the first substance can be crosslinked with thealdehyde.

In further embodiments of the above gels, the gel is formulated so thatit is stable for at least about six months when maintained at atemperature of about 65° C.

In further embodiments of the above described gels the relative amountsof, and the selection of, the first substance, the aldehyde, the wateror water-containing liquid, and the acidic catalyst are adjusted so thatwhen the gel is gelled in the pores of a sandstone core which iscontinuously maintained at 80° C. there will be produced a permeabilityin the core to an aqueous liquid selected from the group consisting ofwater and brine, about 90 days after gellation of the gel in the core,which is no greater than about 15% of the permeability of the core tothe aqueous liquid prior to the gelling of the gel in the core.

In other further embodiments of the above gels, the relative amounts of,and selection of, the first substance, the aldehyde, the water orwater-containing liquid, and the acidic catalyst are adjusted so thatthe gel when gelled in the pores of a sandstone core which iscontinuously maintained at 125° C. will produce a permeability in thecore to an aqueous liquid selected from the group consisting of waterand brine, about 40 days after gellation of the gel in the core, whichis no greater than about 40% of the permeability of the core to theaqueous liquid prior to the gelling of the gel in the core.

In preferred embodiments of the gels, the aldehyde is a dialdehyde. In afurther embodiment the dialdehyde is selected from the group consistingof glyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehydeand mixtures thereof.

In other embodiments of the gels the amount the aldehyde is from about0.005 to about 2.5% of the weight of the gel.

In still further embodiments of the above described gels, the water usedto fom the gel has a hardness of at least about 1000 ppm. in furtherembodiments the water has a hardness of at least about 3000 ppm, or 6000ppm, or higher. In other further embodiments of the above describedgels, the water used to form the gel has a total dissolved solidscontent of at least about 30,000 ppm. In a still further embodiment suchwater has a total dissolved solids content of at least about 80,000 ppm.

In other embodiments the water used to form the gels has a pH betweenabout 2 and about 5. In preferred embodiments, the water used to formthe gel has a pH between about 2.5 and about 4.5. In some embodimentsthe aldehyde of the above gels is at least about 50%wt malonaldehyde andin other embodiments the aldehyde is at least about 50%wtglutaraldehyde.

In other embodiments of the above described gels the first substance isselected from the group consisting of polyvinyl alcohol, a polyvinylalcohol copolymer, and mixtures thereof, which has an average molecularweight of at least about 100,000.

In other embodiments the amount of the first substance is from about 0.5to about 5% of the weight of the gel.

In other embodiments of the gels of this invention, the gel is treatedwith a basic substance, and preferably such treatment neutralizes thegel and/or acidic catalyst used to form the gel, or the immediateenvironment in which the gel is utilized.

In some embodiments of this invention the polyvinyl alcohols and/orpolyvinyl alcohol copolymers of the above described gels are crosslinkedwith aldehydes to form hydrogels containing from about 95 to 98 or 99%water. The aldehydes crosslink the polyvinyl alcohols or polyvinylalcohol copolymers through formation of acetals. It has been found thatgels formed in this way are insensitive to the hardness of the water inwhich they are formed or exposed, or formed from. These gels are alsomore stable at high temperatures than polyacrylamide based gels or gelsmade from biopolymers or polyvinyl alcohols gelled by other crosslinkingagents such as borate.

Because of the insensitivity of these gels to water hardness or totaldissolved solids content, these gels can be prepared using formationwater, brackish water, sea water or usually any other available sourceof water conveniently at hand. Because the largest ingredient used toformulate the above described gels is principly water, substantialeconomic advantage is provided by this invention which permits gels tobe formed with the cheapest source of available water. However, theadvantages of this invention are not limited merely to economicadvantages because these gels also provide substantial technicaladvantages over other gels. For example, in many of their uses thesegels are subjected to the infusion of severely contaminated water intothe gelling mass prior to reaching its gellation point. Where suchcontaminated water infusion occurs in many other gelling fluids thegellation thereof is destroyed or so severely harmed that such othergels, if in fact they do gel, would be rendered ineffective for theirintended use.

Methods of Forming Gels In Situ

This invention also provides for a method of forming, in situ, a gel ina subterranean zone comprising introducing an aqueous solution whichcomprises a first substance selected from the group consisting ofpolyvinyl alcohol, polyvinyl alcohol copolymer, and mixtures thereof,into voids of a subterranean zone; introducing a second substancecomprising an aldehyde into the voids; and forming a gel in the voids bythe reaction of the first substance with the aldehydes in the presenceof an acidic catalyst.

There is also provided by this invention a method of forming, in situ agel in a subterranean zone comprising introducing an aqueous solutionwhich comprises a first substance selected from the group consisting ofpolyvinyl alcohol, polyvinyl alcohol copolymer, and mixtures thereofinto voids of a subterranean zone; introducing a second substancecomprising an aldehydes into the voids; introducing an acidic catalystinto the voids; and forming a gel in said voids by the reaction of thefirst substance with the second substance in the presence of the acidiccatalyst.

In further embodiments of the above described methods, the aqueoussolution has a pH between about 2 and about 5, and in preferredembodiments the pH is between about 2.5 and about 4.5. Acidic pH'soutside these ranges can be used; however, the rate is usually too fastor too slow for practical application. In further embodiments the acidiccatalyst is a Bronsted acid and in other embodiments the acidic catalystis a Lewis acid. In other embodiments the acidic catalyst is a delayedaction catalyst. In still further embodiments of the above describedmethods the aqueous solution and the second substance are introducedinto the voids in one time period and the acidic catalyst is introducedinto the voids in another time period which is different from the firstmentioned time period. In other embodiments of such methods the aqueoussolution is introduced into the voids in one time period, and the secondsubstance and the acidic catalyst are introduced into the voids inanother time period which is different than the first mentioned timeperiod. In still alternate embodiments of such methods the aqueoussolution, the second substance, and the acidic catalyst are mixedtogether before introducing them into the voids. In still otherembodiments of such methods the acidic catalyst is a delayed actioncatalyst.

In other embodiments the subterranean zone in which the gel is formedhas a temperature of at least about 80° C., and in other embodiments atleast about 125° C.

The above described methods of forming a gel in situ in subterraneanformations can be practiced using all of the gels provided by thisinvention.

Processes for Retarding Water Flow

This invention also provides for a process for retarding the flow ofwater in a subterranean zone comprising treating a subterranean zonewith an effective amount of a gel operable for retarding the flow ofwater in the zone, the gel in the presence of an acidic catalyst beingformable from an aqueous solution comprising a first substance selectedfrom the group consisting of polyvinyl alcohol, a polyvinyl alcoholcopolymer, and mixtures thereof, and a second substance comprising analdehyde.

There is also provided by this invention a process for retarding theflow of water in a subterranean zone comprising treating the zone withan effective amount of a gel under conditions which are operable forretarding the flow of water in the zone, the gel being formed in thepresence of an acidic catalyst by reacting an aqueous solutioncomprising a first substance selected from the group consisting ofpolyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof,and a second substance comprising an aldehyde, and allowing the aqueoussolution and the aldehyde to react in the zone in the presence of theacidic catalyst to form a gel in the zone which is effective forretarding the flow of water in the zone.

In still another embodiment there is provided a process for improvingthe production of hydrocarbons and retarding the production of waterfrom a subterranean hydrocarbon-producing zone comprising treating asubterranean water-conveying zone which is in water communication with asubterranean hydrocarbon-producing zone with an effective amount of agel operable for retarding the flow of water in the subterraneanwater-conveying zone, the gel being formed, in the presence of an acidiccatalyst, by reacting an aqueous solution comprising a first substanceselected from the group consisting of polyvinyl alcohol, a polyvinylalcohol copolymer, and mixtures thereof, and a second substancecomprising an aldehyde. This particular embodiment further comprisesallowing the aqueous solution and the aldehyde to react in thesubterranean water-conveying zone to form a gel in such zone which isoperable for retarding the flow of water in such zone and retarding theproduction of water from the hydrocarbon-producing zone, the producinghydrocarbons and retarding the production of water from the subterraneanhydrocarbon-producing zone. The principles of this invention can be usedwhere the subterranean water-conveying zone is under the subterraneanhydrocarbon-producing zone; or where the subterranean water-conveyingzone surrounds the subterranean hydrocarbon-producing zone; or where atleast part of the subterranean water-conveying zone coincides with atlease part of the subterranean hydrocarbon-producing zone.

Still further the embodiments of the above described processes, can beused where the subterranean zone has a temperature of at least about 65°C.; or where such temperature is at least about 80° C., or even moreadvantageously relative to other commonly employed gels where suchtemperature is at least about 125° C.

In some embodiments the aqueous solution of the above describedprocesses is formed from water having a hardness of at least about 1000ppm. In some embodiments the aqueous solution of the above describedprocesses is formed from water having a total dissolved solids contentof at least about 30,000 ppm. In other embodiments, the aqueous solutionused in the above described processes has a pH between 2 and about 5. Inother embodiments of the above described processes the first substancehas an average molecular weight of at least about 100,000.

In one embodiment of this invention directed to a water flood operation,it frequently is desirable to treat the water injector wells with apolymer gel forming solution to control the water flow profile. In thisembodiment such treatment prevents channeling of water at the injectorwell and/or controls and/or redirects water flow through regions ofvarying permeability. Since in this embodiment the polymer is injectedas a relatively low viscosity aqueous phase it penetrates preferentiallythe region of highest permeability to water. Accordingly, afterpolymerization of the gel in high permeability regions, such regions areconverted to low permeability to further retard water flow therebycausing, upon further water injection, a water sweep of previouslyinaccessible areas in the formation which usually have relatively lowpermeability. By extending the water flow to such previouslyinaccessible regions, more hydrocarbons can be recovered than would berecovered in the absence of such polymer treatment.

The above described processes for retarding the flow of water insubterranean zones can be practiced with all of the gels provided bythis invention.

Where the gels of this invention are used to retard or block the flow ofsubterranean water associated with the production of hydrocarbons fromwells it is preferable to form the gel from as high a molecular weightpolyvinyl alcohol or polyvinyl alcohol copolymer as practical, tocrosslink with a dialdehyde having as long a chain of carbon atoms aspractical, to form as few crosslinks as necessary to hold the geltogether, and to maximize the amount of water used to form the gel. Forexample where the gel meets one or more of the following specifications:

The average molecular weight of the polyvinyl alcohol or a copolymerthereof is at least about 100,000.

The polyvinyl alcohol or copolymer thereof provides about 0.5 to about5% of the weight of the gel.

The aldehyde is a dialdehyde selected from the group consisting ofglyoxal, malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde,and mixtures thereof.

The aldehyde provides about 0.005 to about 1.5% of the weight of thegel.

The amount of the aldehyde is such that stoichiometrically no more thanabout 2% or about 1% of the crosslinkable sites of the polyvinyl alcoholor copolymer thereof can be crosslinked with the aldehyde.

The water of the gel-forming composition provides about 98% or about 99%of the weight of the gel.

Gels meeting one or more of the above described specifications arestretchable, elastic and relatively stable in high temperature formationhaving high water hardness levels.

The gels of this invention have improved resistance to heat and arestable in hard water. These characteristics make these gels particularlyuseful for many oil field applications such as water mobility control.These gels may be advantageously used in other harsh environments suchas solar pond construction where they can be used to consolidate loosesoil and to retard or stop the leakage of brine through the pond floor,or to prevent convective flow of the lower, hotter water into the upper,cooler water layer(s). For oil field application, no other gels areknown which exhibit the stability and durability of the gels of thisinvention.

Accordingly, one objective of this invention is to provide a means ofcontrolling water movement in oil wells and subterranean formationsespecially in formations having temperatures 80° C. or higher, or wherethe waters involved are saline or hard.

Another object of this invention is to provide a means to thicken or gelwater with an inexpensive polymer for other oil field developmental usessuch as fracture fluids and fluids for secondary and tertiary oilrecovery. It is another object of this invention to provide a gel whichcan be formulated using hard water and water containing a high level ofdissolved solids such as sea water and formation water encountered indeep off-shore hydrocarbon fields.

Another object of this invention is to provide a gel which is stable athigh temperatures and in particular more stable than other gels at suchhigh temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the percent of original permeability of sandstonecores after their treatment according to the precepts of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An oil well having an undesirable amount of water production is treatedby injecting a polyvinyl alcohol solution containing 1 to 50,000 ppm ofpolyvinyl alcohol having an average molecular weight of 100,000 orhigher. This is followed by injection of a crosslinking agent such asglyoxal with an acidic catalyst. The polymer will undergo crosslinkingand gel in situ in a period of time ranging between several hours toseveral days depending upon, in part, the temperature and amount ofacidic catalysts. The following examples demonstrate how some of thegels of this invention can be made and how such gels are effective inreducing the permeability of sandstone materials to the flow of brines.

EXAMPLES

The following examples demonstrate the procedures by which polyvinylalcohol (PVA) solutions can be gelled using malonaldehydebisdimethylacetal as crosslinking agent and sulfuric or acetic acid ascatalyst. In this example and all later examples calling for 2.5 wt% PVAin water or brine, a stock solution was prepared first by slowly addingthe appropriate weighed amount of dry PVA powder to the water at roomtemperature with stirring and then, with continued stirring, raising thetemperature to 100° C. in a hot water bath. A clear homogeneous solutionresulted after 30 minutes at 100° C. at which time the solution wasallowed to cool to room temperature. The polyvinyl alcohol was (unlessotherwise stated) 98% hydrolyzed, 126,000 molecular weight polyvinylalcohol (Aldrich Chemical Co.) or Elvanol HV 99% hydrolyzed polyvinylalcohol (DuPont).

As used herein, Brine A refers to a synthetic brine prepared by addingthe following amounts of salts to deionized water and adjusting thevolume to 1 liter:

    ______________________________________                                        NaCl                75.5   gr,                                                NaHCO.sub.3         1.2    gr,                                                MgSO.sub.4.7H.sub.2 O                                                                             1.1    gr,                                                MgCl.sub.2.6H.sub.2 O                                                                             3.56   gr, and                                            CaCl.sub.2.2H.sub.2 O                                                                             20.76  gr.                                                ______________________________________                                    

As used herein, Brine B refers to a synthetic brine prepared by addingthe following amounts of salts to deionized water and adjusting thevolume to 1 liter:

    ______________________________________                                        NaCl                61.02  gr,                                                KCl                 1.39   gr,                                                CaCl.sub.2.2H.sub.2 O                                                                             9.20   gr,                                                MgCl.sub.2.6H.sub.2 O                                                                             5.77   gr, and                                            BaCl.sub.2.2H.sub.2 O                                                                             0.02   gr.                                                ______________________________________                                    

As used herein, Brine C refers to a synthetic brine prepared by addingthe following amounts of salts to deionized water and adjusting thevolume to 1 liter:

    ______________________________________                                        NaCl                75.5   gr,                                                MgSO.sub.4.7H.sub.2 O                                                                             1.1    gr,                                                MgCl.sub.2.6H.sub.2 O                                                                             3.56   gr, and                                            CaCl.sub.2.2H.sub.2 O                                                                             20.76  gr.                                                ______________________________________                                    

EXAMPLE NO. 1

0.01 ml malonaldehyde bisdimethylacetal was added to 5 ml of a 2.5 wt%solution of PVA in deionized water. This solution was acidified byadding 0.05 ml of concentrated H₂ SO₄. After 3.25 hrs at roomtemperature this solution had gelled to a cloudy white solid. After fourhrs a small amount of water began to separate from the gel.

EXAMPLE NO. 2

0.01 ml malonaldehyde bisdimethylacetal was added to 5 ml 2.5 wt% PVA indeionized water. This solution was acidified by adding 0.05 ml aceticacid. The sample was then maintained, in a capped glass vial, at 95° C.A pale white gel formed after 1.75 hrs at room temperature.

EXAMPLE NO. 3

The following example illustrates the effect of neutralizing the acidiccatalyst after the gel has formed, on gel stability.

Two sample vials were each charged with 5 ml 2.5 wt% PVA in deionizedwater. To each was added 0.005 ml of malonaldehyde bisdimethylacetal and0.05 ml of 1.8M sulfuric acid. The vials were then capped and placed ina 95° C. oven. After 1 hour one of the vials was removed from the ovenand treated with 48 mg NaHCO₃ in 2 ml H₂ O for 16 hrs, after which thesolution was decanted off and the remaining clear gel was rinsed withthree 5 ml portions of deionized water. The gel was returned to theoven. After one week at 95° C. the gel which had been treated withNaHCO₃ remained clear. The sample which had not been treated with NaHCO₃had clouded within 24 hrs and disintegrated within one week.

EXAMPLE NOS. 4 TO 8 Gelation of Polyvinyl Alcohol with Glutaraldehyde

A stock solution of 2.5 wt% polyvinyl alcohol was prepared in Brine A.To 20 ml of this solution was added 0.5 ml acetic acid, 0.02 ml of a 25wt% solution of glutaraldehyde in water. The pH was then adjusted to thedesired value with sodium acetate solution (0.344 gr NaC₂ H₃ O₂.3H₂ Oper ml Brine A). The resulting solution was mixed thoroughly and placedin a stoppered container in a constant temperature bath at 80° C. Thegel point, characterized by an abrupt increase in viscosity, wasdetermined by monitoring the viscosity with a Brookfield Viscosimeter ornoting visually the resistance to stirring with a magnetic stir bar.Each test for Example Nos. 4 to 8 was conducted entirely at 80° C. Testparameters and gel times are shown in Table I.

                  TABLE I                                                         ______________________________________                                        CONCENTRATION (wt. %)                                                                                                    Gel                                Example       Glutar-  Acetic NaC.sub.2 H.sub.3 O.sub.2                                                                  Time                               No.    PVA    aldehyde Acid   .3H.sub.2 O                                                                           pH   (min)                              ______________________________________                                        4      2.4    0.025    2.3    2.      3.54 70.5                               5      2.5    0.025    2.3    0.6     3.33 40                                 6      2.5    0.025    2.4    0.25    3.00 24                                 7      2.5    0.025    2.4    0.17    2.90 18                                 8      2.5    0.025    2.5    0.10    2.71 13                                 ______________________________________                                    

EXAMPLES NOS. 9 TO 11

Example Nos. 9 to 11 demonstrate the effect of temperature on gel time.

A stock solution of 2.4 wt% PVA was prepared in synthetic Brine A. To 20ml of this solution was added 0.5 ml of acetic acid, 0.04 ml ofmalonaldehyde bisdimethylacetal, and enough of a sodium acetate solution(0.344 gr NaC₂ H₃ O₂ 3H₂ O per 1 ml Brine A) to bring the pH of thesolution to 2.9. The resulting solution was mixed thoroughly and placedin a stoppered vial in a constant temperature bath at the temperatureshown in Table II. The gel times were determined as in Example Nos. 4 to8.

                  TABLE II                                                        ______________________________________                                        Example                                                                       No.           T (°C.)                                                                        Gel Time (min)                                          ______________________________________                                         9            65      162                                                     10            95      31                                                      11            127     13.5                                                    ______________________________________                                    

Example Nos. 12 to 18 illustrate the use of delayed acidic actioncatalysts.

EXAMPLE NO. 12

A mixture of 20 ml of 2.5 wt% PVA in Brine A, 0.005 ml of glutaraldehydeand 79 mg of ZnCl₂ in 1 ml of Brine A was placed in a stoppered vial andkept at 180° F. After 21 hrs the solution had not gelled. An additional30 mg ZnCl₂ was added. After 141 hrs at 180° F. a semi-rigid gel hadformed. Similar experiments using 0.5%, 1% and 2% ZnCl₂ and 2.5% PVA inseawater failed to yield gels after 350 hrs at 180° F.

EXAMPLE NOS. 13 TO 16

In Example Nos. 13 to 16, the samples were prepared with a total volumeof 20 ml using the following wt% concentrations.

    ______________________________________                                        CONCENTRATION (wt. %)                                                         Example        Methyl   Glutar- Water  Gel Time                               No.    PVA     Formate  aldehyde                                                                              Source (hrs)                                  ______________________________________                                        13     2.5     0.144    0.1     Tap    29                                     14     2.5     0.048    0.1     Tap    144                                    15     2.5     0.144    0.1     Brine B                                                                              21                                     16     2.5     0.048    0.1     Brine B                                                                              21                                     ______________________________________                                    

After 5 hrs at 180° F., the solutions had not thickened perceptibly.After 21 hrs the samples prepared with Brine B had gelled. The samplesin tap water took slightly longer to form and were not as rigid as thosein Brine B.

EXAMPLE NOS. 17 AND 18

To 10 ml of a solution of 2.5 wt% PVA in Brine B was added 0.007 ml(0.075 wt% of the total mixture) of glutaraldehyde and 0.015 ml (0.18wt%of the total mixture) of glycerol monoacetate. A second solution wasmade substituting 0.015 ml of glycerol diacetate for the glycerolmonoacetate. The solutions were held at 180° F. After 20 hr there was noobservable change. After 25 hrs, both had formed very soft, flowinggels. After 92 hrs both had formed rigid elastic gels, and some waterhad separated.

EXAMPLE NOS. 19 AND 20

These Examples demonstrate the use of polyvinyl alcohol gels to reducethe permeability of porous media to brine.

Core Sample Preparation Procedure

A sandstone core about 3 inches long by 1.5 inches in diameter was firstsaturated with brine and then mounted in a core holder similar tocommercially available core holders which are known in the arts such asthose sold by Core Laboratories, Inc., in Dallas, Tex. Such core holdershave a tightly fitting expandable sleeve for extending entirely over thecylindrical length of the core and beyond the ends of the core. Thesleeve is used to attach and mount the core in the core holderapparatus. The core holder was placed in an oven, equilibrated to thedesired test temperature, and connected to a system operable forinfusing a gel forming fluid into the pores of the core. The pressuredrop across the length of the core was measured during its treatmentwith the gel forming fluid.

To simulate a formation in which oil has been displaced by water, as forexample in a subterranean oil producing formation, after beingequilibrated to test temperature, the core was first infused withkerosene and then infused with brine to simulate subterraneanenvironments encountered. The brine and the test temperature should beselected so as to best simulate the particular conditions of theformation of interest. Conditioning of the core in this manner isreferred to herein as "residual oil formation simulation".

When residual oil formation simulation had been established in the core,its permeability to brine was determined using Darcy's law by measuringthe pressure drop across the length of the core for various brine flowrates. The calculations were made as follows: ##EQU1## K is thepermeability of the core in millidarcies, l is the length of the core incm,

u is the viscosity of brine in cp,

q is the flow rate of brine in cc/sec,

p is the pressure drop across the length of the core in atm, and

A=cross section of the core in cm².

After forming a test gel in the pores of the core, the permeability ofthe core is substantially reduced. This is evidenced by an increasedpressure drop across the core relative to that existing before gelation.Using Darcy's equation, the new permeability can be expressed as apercent of the original permeability as follows, where ##EQU2## is the %of original permeability, K_(t) is the permeability at time t after gelformation, and

K_(o) is the original permeability.

The duration of the gels effectiveness, which is referred herein as itsstability, can be monitored by determining K_(t) as a function of time.During testing the core is continually maintained at the temperature ofinterest.

To prevent the gel from setting up in the apparatus, the test procedureadopted first pumped into the core a non-gelling composition, followedby the test composition, which was followed by a small quantity ofadditional non-gelling compositions. Since such fluid flowed through thecore in plug like flow, the quantities of fluid were controlled so thatthe gelling fluid remained only in the core, with non-gellingcomposition in the apparatus and slightly in each end of the core.

The Example Nos. 19 and 20, which follow, show the actual reduction inpermeability which was experienced after forming a PVA-aldehyde gel inthe pores of sandstone cores.

EXAMPLE NO. 19, GEL FORMATION AT 127° C. (260° F.)

After equilibration to 127° C., a Berea sandstone core, 3 inches longand 1.5 inches in diameter was conditioned for residual oil formationsimulation, using Brine C, as described above. A aliquot of gel formingfluid equal to 0.70 pore volume (14.9 cc) was infused into the corewhich was then followed by an infusion of 0.20 pore volume (4.3 cc) ofBrine C. In this manner the gel forming fluid was infused into thecenter part of the core, while each end of the core (about 0.10 porevolume on one end and about 0.20 pore volume on the other end of thecore) remained essentially free of the gel forming fluid. Flow wasstopped for 4 hours and the gel was then allowed to form in the pores.Thereafter, intermittent infusion with Brine C was conducted over thenext 50 days to determine the permeability of the thusly "gelled" coreto Brine C. The results are given in Table III and shown in FIG. 1wherein the permeability of the core is expressed as the percent of theoriginal core permeability as a function of time.

In this Example, the gel forming fluid was prepared by combining 100 ccof 2.5 wt% with polyvinyl alcohol in Brine C, 1 cc of glacial aceticacid, 0.365 gr of sodium acetate trihydate, and 0.20 ml ofmalonaldehyde-bisdimethylacetal. The pH of the initial gel forming fluidwas 3.4.

EXAMPLE NO. 20, GEL FORMATION AT 82° C. (180° F.)

Another Berea sandstone core was equilibrated to 82° C. and conditionedfor residual oil formation simulation using Brine B, as described above.An aliquot of gel forming fluid equal to 0.75 pore volume (13.7 ml) wasinfused into the pores of the core. The gel forming fluid in the linesleading to the core was then forced into the core by displacement with7.0 ml of 2.5% polyvinyl alcohol. The flow was stopped for 3 hours topermit the gel to form. Thereafter, intermittent brine infusion over thenext 3 months was conducted to determine the permeability of the thusly"gelled" core to Brine B.

The gel forming fluid for this Example was prepared by combing 100 ml of2.5 wt% polyvinyl alcohol in Brine B and 0.125 ml of 25% aqueousglutaraldehyde followed by adjustment of the pH to 2.7 with glacialacetic acid (0.8 ml). The results of this experiment are also given inTable IV and shown in FIG. 1.

Unless otherwise specified herein all percents are weight percents.

The gels, the methods of forming the gels, and the processes forretarding the flow of water have some degree of flexibility. Forexample, if the environment in which the gels are to be used has arelatively high temperature, gel time can be slowed by using a smalleramount of acidic catalyst. Similarly, if the environmental temperatureis relatively low, gellation can be speeded by the use of larger amountsof acidic catalyst. If it is desirable to use the formation water in asubterranean zone as part of the final gel, a delayed action catalystcan be used to allow for the diffusion of such formation water into theinjected gel-forming fluids so that such formation water will becomepart of the gel. For water blocking in subterranean zones it ispreferred to use a relatively small amount of aldehyde so that thedegree of crosslinking is relatively small because gels with low degreesof crosslinking have been found to be more stable than gels with arelatively high degree of crosslinking. Other variations offormulations, methods and processes will be apparent from this inventionto those skilled in the art.

The foregoing disclosure and description of the present invention isillustrative and explanatory thereof and various changes in gelformation procedures and gel composition as well as the uses andapplications of such gels to form them in situ in subterranean zones andto retard or block water in subterranean zones may be made within thesclpe of the appending claims without departing from the spirit of theinvention. For example, many gel formulations can be produced and manymethods of forming such gels in situ in subterranean deposits will beapparent to one skilled in the art from this invention. For example, anynumber of sequential injection steps of the gel forming fluids can bemade. Furthermore, the necessary concentrations, amounts and sequence ofinjection of the gel forming fluids can be tailored to suit theparticular well or subterranean formation being treated.

                  TABLE III                                                       ______________________________________                                        (Example No. 19)                                                              Gel Formation at 127° C. (260° F.)                              Gel Forming Fluid Composition (wt. %)                                         98.51       2.5 wt % polyvinyl alcohol in Brine C                             0.97        Glacial acetic acid                                               0.34        Sodium acetate trihydrate                                         0.18        Malonaldehyde-bisdimethylacetal                                   % of Initial Permeability                                                     vs                                                                            Time Lapsed After Gellation                                                                  Permeability                                                                  (% of initial permeability                                     Time           before treatment with gel                                      (days after gellation)                                                                       forming fluids)                                                ______________________________________                                        1              6.1                                                            2              13.1                                                           3              16.9                                                           6              19.8                                                           7              21.3                                                           8              17.2                                                           9              20.8                                                           10             18.0                                                           13             21.9                                                           15             21.9                                                           16             22.5                                                           17             25.3                                                           20             25.3                                                           35             32                                                             36             25                                                             37             26                                                             38             28                                                             41             27                                                             42             29                                                             43             85                                                             45             70                                                             ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        (Example No. 20)                                                              Gel Formation at 82° C. (260° F.)                               Gel Forming Fluid Composition (wt %)                                          99.13       2.5 wt % polyvinyl alcohol in Brine C                             0.12        25% aqueous glutaraldehyde                                        0.75        Glacial acetic acid                                               % of Initial Permeability                                                     vs                                                                            Time Lapsed After Gellation                                                                  Permeability                                                                  (% of Initial permeability                                     Time           before treatment with gel                                      (days after gellation)                                                                       forming fluids)                                                ______________________________________                                        1              1.3                                                            2              1.3                                                            3              1.8                                                            4              1.7                                                            8              2.8                                                            9              2.8                                                            10             2.0                                                            14             2.5                                                            15             2.7                                                            17             4.4                                                            22             5.3                                                            23             4.9                                                            24             4.7                                                            25             9.5                                                            29             7.8                                                            31             7.5                                                            35             8.3                                                            37             7.3                                                            39             9.6                                                            42             8.1                                                            43             9.5                                                            44             7.8                                                            46             9.3                                                            49             11.9                                                           50             11.6                                                           56             7.1                                                            57             7.8                                                            58             9.6                                                            59             9.1                                                            63             5.4                                                            64             9.8                                                            65             8.0                                                            66             8.0                                                            70             9.2                                                            72             10.2                                                           73             9.8                                                            74             9.2                                                            78             9.6                                                            80             9.2                                                            81             4.9                                                            84             4.7                                                            87             4.9                                                            89             5.7                                                            94             5.9                                                            99             5.7                                                            102            5.2                                                            ______________________________________                                    

I claim:
 1. A process for retarding the flow of water in a subterraneanzone comprising introducing an effective amount of a gel-formingcomposition into a subterranean zone, said gel-forming composition beingoperable when gelled in said zone for retarding the flow of watertherein, said gel-forming composition being formable into a gel in thepresence of an acidic catalyst fromi. an aqueous solution comprising afirst substance selected from the group consisting of polyvinyl alcohol,a polyvinyl alcohol copolymer, and mixtures thereof, and ii. a secondsubstance comprising an aldehyde.
 2. The process of claim 1, whereinsaid gel-forming composition comprises water, and wherein said waterprovides at least about 95% of the weight of said gel.
 3. The process ofclaim 2, wherein said water provides at least about 96% of the weight ofsaid gel.
 4. The process of claim 2, wherein said water provides atleast about 97% of the weight of said gel.
 5. The process of claim 2,wherein said water provides at least about 98% of the weight of saidgel.
 6. The process of claim 2, further comprising treating said gelwith a basic substance.
 7. The process of claim 2, wherein said waterhas a hardness of at least about 1000 ppm.
 8. The process of claim 2,wherein said first substance has an average molecular weight of at leastabout 100,000.
 9. The process of claim 2, wherein said water has a pHbetween about 2.5 and about 4.5.
 10. The process of claim 1, whereinsaid first substance isa polyvinyl alcohol having an average molecularweight of at least about 100,000, wherein said polyvinyl alcohol isabout 0.5 to about 5% of the weight of said gel, and wherein saidaldehyde is a dialdehyde selected from the group consisting of glyoxal,malonaldehyde, succinaldehyde, glutaraldehyde, adipaldehyde, andmixtures thereof, wherein said dialdehyde is about 0.005 to about 2.5%of the weight of said gel.
 11. The process of claim 10, wherein saiddialdehyde is malonaldehyde.
 12. The process of claim 10, wherein saiddialdehyde is glutaraldehyde.
 13. The process of claim 1, whereinsaidfirst substance has crosslinkable sites and wherein the amount ofsaid aldehyde is adjusted so that stoichiometrically no more than about8% of said crosslinkable sites of said first substance can becrosslinked with said aldehyde.
 14. The process of claim 13, whereinsaid amount of said aldehyde is adjusted so that stoichiometrically nomore than about 4% of said crosslinkable sites of said first substancecan be crosslinked with said aldehyde.
 15. The gel of claim 13, whereinsaid amount of said aldehyde is adjusted so that stoichiometrically nomore than about 2% of said crosslinkable sites of said first substancecan be crosslinked with said aldehyde.
 16. The gel of claim 13, whereinsaid amount of said aldehyde is adjusted so that stoichiometrically nomore than about 1% of said crosslinkable sites of said first substancecan be crosslinked with said aldehyde.
 17. The gel of claim 13, whereinsaid water has a hardness of at least about 1000 ppm.
 18. The gel ofclaim 13, wherein said water has a pH between about 2.5 and 4.5.
 19. Thegel of claim 13, wherein said aldehyde is at least about 50 wt%malonaldehyde.
 20. The gel of claim 13, wherein said aldehyde is atleast about 50 wt% glutaraldehyde.
 21. The process of claim 13, furthercomprising treating said gel with a basic substance to neutralize saidgel.
 22. The process of claim 1, wherein said first substance isapolyvinyl alcohol having crosslinkable sites, and wherein said aldehydeis a dialdehyde, the amount of said dialdehyde being adjusted so thatstoichiometrically no more than about 8% of said crosslinkable sites ofsaid polyvinyl alcohol can be crosslinked with said dialdehyde.
 23. Theprocess of claim 1, wherein said first substance isa polyvinyl alcoholhaving crosslinkable sites and an average molecular weight of at leastabout 100,000, wherein said polyvinyl alcohol is about 0.5 to about 5%of the weight of said gel, and wherein said aldehyde is a dialdehydeselected from the group consisting of glyoxal, malonaldehyde,succinaldehyde, glutaraldehyde, adipaldehyde, and mixtures thereof,wherein said dialdehyde is about 0.005 to about 2.5% of the weight ofsaid gel, the amount of said dialdehyde being adjusted so thatstoichiometrically no more than about 8% of said crosslinkable sites ofsaid polyvinyl alcohol can be crosslinked with said dialdehyde.
 24. Theprocess of claim 23, wherein said dialdehyde is malonaldehyde.
 25. Theprocess of claim 23, wherein said dialdehyde is glutaraldehyde.
 26. Theprocess of claim 1, wherein said acidic catalyst is selected from thegroup consisting of acetin, diacetin, and mixtures thereof.
 27. Aprocess for retarding the flow of water in a subterranean zonecomprising:(a) introducing an effective amount of a gel-formingcomposition into a subterranean zone, said gel-forming composition beingoperable when gelled in said zone for retarding the flow of watertherein, said gel-forming composition being formed, in the presence ofan acidic catalyst, into a gel by reactingi. an aqueous solutioncomprising a first substance selected from the group consisting ofpolyvinyl alcohol, a polyvinyl alcohol copolymer, and mixtures thereof,and ii. a second substance comprising an aldehyde; and (b) allowing saidaqueous solution and said aldehyde to react in said zone and to form agel therein which is effective for retarding the flow of water in saidzone.
 28. The process of claim 27, wherein said subterranean zone has atemperature of at least about 65° C.
 29. The process of claim 27,wherein said subterranean zone has a temperature of at least about 80°C.
 30. The process of claim 27, wherein said subterranean zone has atemperature of at least about 125° C.
 31. The process of claim 27,wherein said aqueous solution is formed from water having a hardness ofat least about 1000 ppm.
 32. The process of claim 27, wherein saidgel-forming composition compriseswater, and wherein said water providesabout 95 to about 99% of the weight of said gel.
 33. A process forretarding the flow of water while producing hydrocarbons from asubterranean hydrocarbon-producing zone comprising:(a) introducing aneffective amount of a gel-forming composition into a subterraneanwater-conveying zone, which is in water communication with asubterranean hydrocarbon-producing zone, said gel-forming compositionbeing operable when gelled in said subterranean water-conveying zone forretarding the flow of water therein, said gel-forming composition beingformed, in the presence of an acidic catalyst, into a gel by reactingi.an aqueous solution comprising a first substance selected from the groupconsisting of a polyvinyl alcohol, a polyvinyl alcohol copolymer, andmixtures thereof, and ii. a second substance comprising an aldehyde; (b)allowing said aqueous solution and said aldehyde to react in thepresence of said acidic catalyst in said subterranean water-conveyingzone to form a gel therein which is operable for retarding the flow ofwater in said subterranean water-conveying zone and retarding theproduction of water when producing hydrocarbons from saidhydrocarbon-producing zone; and (c) producing hydrocarbons and retardingthe production of water from said subterranean hydrocarbon-producingzone.
 34. The process of claim 33, wherein said subterraneanwater-conveying zone is under said subterranean hydrocarbon-producingzone.
 35. The process of claim 33, wherein said subterraneanwater-conveying zone surrounds said subterranean hydrocarbon-producingzone.
 36. The method of claim 35, wherein said subterranean zone has atemperature of at least about 125° C.
 37. The method of claim 35,wherein said catalyst is a Bronsted acid.
 38. The method of claim 35,wherein said catalyst is a Lewis acid.
 39. The process of claim 33,wherein at least a part of said subterranean water-conveying zonecoincides with at least a part of said subterraneanhydrocarbon-producing zone.
 40. The process of claim 33, wherein saidsubterranean water-conveying zone has a temperature of at least 65° C.41. The process of claim 33, wherein said subterranean water-conveyingzone has a temperature of at least about 80° C.
 42. The process of claim33, wherein said subterranean water-conveying zone has a temperature ofat least about 125° C.
 43. The process of claim 33, wherein said aqueoussolution is formed from water having a hardness of at least about 1000ppm.
 44. A process for retarding the flow of water while producinghydrocarbons from a subterranean hydrocarbon-producing zonecomprising:(a) introducing an effective amount of a gel-formingcomposition into a subterranean water-conveying zone, which is in watercommunication with a subterranean hydrocarbon-producing zone, saidgel-forming composition being operable when gelled in said subterraneanwater-conveying zone for retarding the flow of water therein, saidgel-forming composition being formed, in the presence of an acidiccatalyst, into a gel by reactingi. an aqueous solution of a polyvinylalcohol, and ii. a second substance comprising a dialdehyde; (b)allowing said aqueous solution and said dialdehyde to react, in thepresence of said acidic catalyst, in said subterranean water-conveyingzone to form a gel therein which is operable for retarding the flow ofwater in said subterranean water-conveying zone and retarding theproduction of water when producing hydrocarbons from saidhydrocarbon-producing zone, wherein the water of said gel-formingcomposition provides about 95 to about 99% of the weight of said gel;(c) producing hydrocarbons and retarding the production of water fromsaid subterranean hydrocarbon-producing zone.
 45. A process forretarding the flow of water and producing hydrocarbons from asubterranean hydrocarbon-producing zone comprising:(a) introducing aneffective amount of a gel-forming composition into a subterraneanwater-conveying zone, which is in water communication with asubterranean hydrocarbon-producing zone, said gel-forming compositionbeing operable when gelled in said subterranean water-conveying zone forretarding the flow of water therein, said gel-forming composition beingformed, in the presence of an acidic catalyst, into a gel by reactingi.an aqueous solution of a polyvinyl alcohol having an average molecularweight of at least 100,000, and ii. a second substance comprising adialdehyde selected from the group consisting of glyoxal, malonaldehyde,succinaldehyde, glutaraldehyde, adipaldehyde, and mixtures thereof; (b)allowing said aqueous solution and said dialdehyde to react, in thepresence of said acidic catalyst, in said subterranean water-conveyingzone to form a gel therein which is operable for retarding the flow ofwater in said subterranean water-conveying zone and retarding theproduction of water when producing hydrocarbons from saidhydrocarbon-producing zone, wherein the amount of said polyvinyl alcoholis about 0.5 to about 5% of the weight of said gel, wherein the amountof said dialdehyde is about 0.005 to about 2.5% of the weight of saidgel, and wherein the water of said gel-forming composition providesabout 95 to about 99% of the weight of said gel; and (c) producinghydrocarbons and retarding the production of water from saidsubterranean hydrocarbon-producing zone.
 46. The process of claim 45,wherein said dialdehyde is malonaldehyde.
 47. The process of claim 45,wherein said dialdehyde is glutaraldehyde.
 48. The process claim 45,wherein said acidic catalyst is selected from the group consisting ofacetin, diacetin, and mixtures thereof.
 49. A method for retarding theflow of water by forming, in situ, a gel in a subterranean zonecomprising:(a) introducing an aqueous solution which comprises a firstsubstance selected from the group consisting of polyvinyl alcohol,polyvinyl alcohol copolymer, and mixtures thereof, into the voids of asubterranean zone; (b) introducing a second substance comprising analdehyde into said voids; and (c) forming a gel in said voids by thereaction of said aqueous solution with said second substance in thepresence of an acidic catalyst thereby retarding the flow of water insaid subterranean zone.
 50. The method of claim 49, wherein said aqueoussolution has a pH between about 2 and about
 5. 51. The method of claim49, wherein said subterranean zone has a temperature of at least about80° C.
 52. A method for retarding the flow of water by forming, in situ,a gel in a subterranean zone comprising:(a) introducing an aqueoussolution of a polyvinyl alcohol into the voids of a subterranean zone;(b) introducing a substance comprising a dialdehyde into said voids; and(c) forming a gel in said voids by the reaction of said aqueous solutionwith said substance, in the presence of an acidic catalyst, wherein thewater of said aqueous solution and of said substance provides about 95to about 99% of the weight of said gel thereby retarding the flow ofwater in said subterranean zone.
 53. A process for retarding the flow ofwater by forming, in situ, a gel in a subterranean zone comprising:(a)introducing an aqueous solution of a polyvinyl alcohol into the voids ofa subterranean zone, said polyvinyl alcohol having an average molecularweight of at least about 100,000; (b) introducing a substance comprisinga dialdehyde into said voids, said dialdehyde being selected from thegroup consisting of glyoxal, malonaldehyde, succinaldehyde,glutaraldehyde, adipaldehyde, and mixtures thereof; and (c) forming agel in said voids by the reaction of said aqueous solution with saidsubstance, in the presence of an acidic catalyst, wherein said polyvinylalcohol is about 0.5 to about 5% of the weight of said gel, wherein saiddialdehyde is about 0.005 to about 2.5% of the weight of said gel, andwherein the water of said aqueous solution and said substance providesabout 95 to about 99% of the weight of said gel thereby retarding theflow of water in said subterranean zone.
 54. The process of claim 53,wherein said dialdehyde is malonaldehyde.
 55. The process of claim 53,wherein said dialdehyde is glutaraldehyde.
 56. The process of claim 53,wherein said acidic catalyst is selected from the group consisting ofacetin, diacetin, and mixtures thereof.
 57. A method for retarding theflow of water by forming, in situ, a gel in a subterranean zonecomprising:(a) introducing an aqueous solution which comprises a firstsubstance selected from the group consisting of polyvinyl alcohol,polyvinyl alcohol copolymer, and mixtures thereof, into voids of asubterranean zone; (b) introducing a second substance comprising analdehyde into said voids; (c) introducing a third substance comprisingan acidic catalyst into said voids; and (d) forming a gel in said voidsby the reaction of said aqueous solution with said second substance inthe presence of said third substance thereby retarding the flow of waterin said subterranean zone.
 58. The method of claim 57, wherein saidacidic catalyst is a Bronsted acid.
 59. The method of claim 57, whereinsaid acidic catalyst is a Lewis acid.
 60. The method of claim 57,wherein said acidic catalyst is a delayed action catalyst.
 61. Themethod of claim 57, wherein said aqueous solution and said secondsubstance are introduced into said voids in one time period, and saidthird substance is introduced into said voids in another time periodwhich is different from said first mentioned time period.
 62. The methodof claim 57, wherein said aqueous solution is introduced into said voidsin one time period, and said second substance and said third substanceare introduced into said voids in another time period which is differentfrom said first mentioned time period.
 63. The method of claim 57,further comprising mixing said aqueous solution, said second substance,and said third substance together before introducing them into saidvoids.
 64. The method of claim 63, wherein said acidic catalyst is adelayed action catalyst.
 65. A method for retarding the flow of water byforming, in situ, a gel in a subterranean zone comprising:(a)introducing an aqueous solution of a polyvinyl alcohol into the voids ofa subterranean zone; (b) introducing a second substance comprising adialdehyde into said voids; (c) introducing a third substance comprisingan acidic catalyst into said voids; and (d) forming a gel in said voidsby the reaction of said aqueous solution with said second substance inthe presence of said third substance, wherein the water of said aqueoussolution, said second substance and said third substance provides about95% to about 99% of the weight of said gel thereby retarding the flow ofwater in said subterranean zone.
 66. A process for retarding the flow ofwater by forming, in situ, a gel in a subterranean zone comprising:(a)introducing an aqueous solution of a polyvinyl alcohol into the voids ofa subterranean zone, said polyvinyl alcohol having an average molecularweight of at least about 100,000; (b) introducing a second substancecomprising a dialdehyde into said voids, said dialdehyde being selectedfrom the group consisting of glyoxal, malonaldehyde, succinaldehyde,glutaraldehyde, adipaldehyde, and mixtures thereof; (c) introducing athird substance comprising an acidic catalyst into said voids; and (d)forming a gel in said voids by the reaction of said aqueous solutionwith said second substance, in the presence of said third substance,wherein said polyvinyl alcohol is about 0.5 to about 5% of the weight ofsaid gel, wherein said dialdehyde is about 0.005 to about 2.5% of theweight of said gel, and wherein the water of said aqueous solution, saidsecond substance and said third substance provides about 95 to about 99%of the weight of said gel thereby retarding the flow of water in saidsubterranean zone.
 67. The process of claim 66, wherein said dialdehydeis malonaldehyde.
 68. The process of claim 66, wherein said dialdehydeis glutaraldehyde.
 69. A process for retarding the flow of water byforming, in situ, a gel in a subterranean zone comprising:(a)introducing an aqueous solution of a polyvinyl alcohol into the voids ofa subterranean zone, said polyvinyl alcohol having an average molecularweight of at least about 100,000; (b) introducing a second substancecomprising a dialdehyde into said voids, said dialdehyde being selectedfrom the group consisting of glyoxal, malonaldehyde, succinaldehyde,glutaraldehyde, adipaldehyde, and mixtures thereof; (c) introducing athird substance comprising a delayed action catalyst into said voids,said delayed action catalyst being selected from the group consisting ofacetin, diacetin, and mixtures thereof; and (d) forming a gel in saidvoids by the reaction of said aqueous solution with said secondsubstance, in the presence of said third substance, wherein saidpolyvinyl alcohol is about 0.5 to about 5% of the weight of said gel,wherein said dialdehyde is about 0.005 to about 2.5% of the weight ofsaid gel, and wherein the water of said aqueous solution, said secondsubstance and said third substance provides about 95 to about 99% of theweight of said gel thereby retarding the flow of water in saidsubterranean zone.
 70. The process of claim 69, wherein said dialdehydeis selected from the group consisting of malonaldehyde, glutaraldehyde,and mixtures thereof.